Although the introduction of carbonaceous materials typically decreases the overall Li-storage capacity because of the reduced content of active material (Zn in this case), the pure Zn e[r]
Trang 1Enhanced performance of carbon-free intermetallic zinc titanium
alloy (Zn-Zn x Ti y ) anode for lithium-ion batteries
Quoc Hai Nguyena,b,1, Nguyen Thanh Hunga,1, Sang Joon Parka, Il Tae Kima,**,
Jaehyun Hura,*
a Department of Chemical and Biological Engineering, Gachon University, Seongnam, Gyeonggi, 13120, Republic of Korea
b Department of Chemical Technology, Baria-Vungtau University, Viet Nam
a r t i c l e i n f o
Article history:
Received 19 December 2018
Received in revised form
22 January 2019
Accepted 29 January 2019
Available online 31 January 2019
Keywords:
Zinc
Titanium
Intermetallic alloy
Carbon-free
Anode
Lithium-ion batteries
a b s t r a c t
A carbon-free intermetallic zinc titanium alloy (carbon-free Zn/Ti) anode, comprising active Zn nano-particlesfinely dispersed in ZnxTiy(Zn0.6Ti0.4and Zn3Ti) intermetallic buffer, is prepared via a thermal-treatment followed by a high-energy mechanical milling process for a lithium-ion battery (LIB) anode As
a counter-intuitive phenomenon, without the need of a carbon matrix, the carbon-free Zn/Ti alloy ex-hibits superior cyclic performance (~1064 mAh cm3of volumetric capacity after 350 cycles), a good rate capability (85% capacity retention at 3 A g1compared to its capacity at 0.1 A g1), and a high initial coulombic efficiency (88%) Although the use of hybrid TiC-C matrix as a control sample still affords highly stable cyclic performance and good rate capability, it exhibits a relatively lower capacity than a carbon-free alloy electrode The enhanced performance of carbon-free Zn/Ti anodes for LIBs is owing to the presence of stable and cohesive ZnxTiyintermetallic phases that provide high conductivity and mechanical stability, thereby mitigating the large volume changes of Zn particles during the lithiation/ delithiation processes High-performance Zn-ZnxTiycan be seen as a new promising anode for the next-generation energy storage technology
© 2019 Published by Elsevier Ltd
1 Introduction
Rechargeable lithium-ion batteries (LIBs) remain the most
attractive alternative energy storage system owing to their high
capacity, long cycle life, and good rate capability [1e3] Since their
first commercial application in 1991, their technologies have
continuously evolved to meet the requirements of the rapid growth
of emerging technologies such as mobile electronic devices, hybrid
electric vehicles, and grid-scale energy storage systems Therefore,
it is essential to explore new electrode materials in order to
improve their energy density, rate capability, safety, and
afford-ability [4e6] Although graphite is currently used as a commercial
anode material for LIBs because of its stability and good
conduc-tivity, its energy density limitations (low theoretical specific
ca-pacity ~372 mAh g1), low tap density (<1 g cm3), and the safety
issues associated with its low reaction potential need to be
addressed [7e10] Therefore, numerous studies have been
dedicated to developing new alternative materials in order to overcome the drawbacks of graphite anodes [9,11e17]
Significant efforts have been devoted to studying new materials with high theoretical capacity, such as Si, SiO2, Ge, Sn, Sb, and Zn (specifically categorized as Li alloying/dealloying materials) because of their extremely high theoretical capacities (Si:
4400 mAh g1, SiO2: 1965 mAh g1, Ge: 1600 mAh g1, Sn: 990 mAh
g1, Sb: 660 mAh g1, and Zn: 410 mAh g1[18e29] Of these, its low theoretical gravimetric capacity notwithstanding (~410 mAh
g1), Zn is a promising material owing to the significant benefits of its extremely high tap density (~7.14 g cm3) and low operating voltage (~0.4 vs Li/Liþ), as well as other beneficial features including low cost, natural abundance, and eco-friendliness However, Zn alone typically undergoes large volume changes during Li alloying/ dealloying, which result in opening cracks and pulverization, leading to poor cycling behavior as with alloying/dealloying ma-terials [30e32]
A common approach to resolve these problems is to introduce appropriate amounts of additional carbon-based materials (e.g., amorphous carbon, carbon nanotube, graphite, and TiC-C) into Zn
as they can play the role of a suitable matrix for Zn in such a way that they improve the cycle life because of their high conductivities
* Corresponding author.
** Corresponding author.
E-mail addresses: itkim@gachon.ac.kr (I.T Kim), jhhur@gachon.ac.kr (J Hur).
1 Contributed equally to this work.
Contents lists available atScienceDirect Electrochimica Acta
j o u r n a l h o me p a g e : w w w e l s e v i e r c o m / l o c a t e / e l e c t a c t a
https://doi.org/10.1016/j.electacta.2019.01.182
0013-4686/© 2019 Published by Elsevier Ltd.
Electrochimica Acta 301 (2019) 229e239
Trang 2and excellent mechanical properties [7,27,33] However, the use of
carbon in composite electrodes poses a number of obvious
disad-vantages: (1) introduction of carbonaceous materials typically
de-creases the overall Li-storage capacity because of their lower
theoretical capacity than Zn, (2) its low tap density (<1 g cm3)
leads to a significant reduction in volumetric capacity [34], and (3)
the high initial capacity loss owing to the irreversible reaction of Liþ
ions on the carbon matrix surface [35,36] Therefore, the
develop-ment of a new electrode that does not contain a carbon-based
matrix could overcome these problems; however, to realize both
high capacity and cyclic stability without adding carbon remains a
challenge
One of the strategies to attain this goal is to create an
appro-priate intermetallic compound that can act as a carbon-free matrix
for active materials This new matrix should be almost
electro-chemically inert, but should possess high conductivity and
me-chanical robustness, thereby contributing to cycling stability
toward the active material In this study, we propose a carbon-free
Zn/Ti alloy in which Zn is embedded in a carbon-free intermetallic
Zn-Ti alloy (ZnxTiy) matrix prepared by high energy mechanical
milling (HEMM) The beneficial features of ZnxTiyare as follows: (1)
their strong ZneTi bond (large negative heats of formation)
inevi-tably causes an increase in the bulk modulus and a decrease in the
average atomic volume that enhances phase stability and cohesive
properties, and (2) their high Young's modulus (>100000 MPa)
implies good mechanical stability As a result, we demonstrate the
excellent electrochemical performance of Zn/Ti alloy (Zn-ZnxTiy)
even without the carbon matrix because of the presence of stable
and cohesive ZnxTiy[37] that can effectively buffer the significant
volume expansion of Zn particles during the Li alloying/dealloying
processes, as well as enhance the electronic conductivity In
particular, at the optimum atomic ratio (1:1) between Zn and Ti,
carbon-free Zn/Ti anodes (Zn/Ti (11)) exhibit excellent cyclic
per-formance (~1064 mAh$cm3 of volumetric capacity after 350
cy-cles), good rate capability (85% capacity retention at 3 A g1
compared to the capacity at 0.1 A g1), and high initial coulombic
efficiency (88%) In comparison, although Zn/Ti (11) with carbon
(namely, Zn/Ti (11)-C or Zn-TiC-C (resulting composite)) also
ex-hibits good stability in cyclic performance, its capacity is signi
fi-cantly lower because of the presence of carbon
2 Experimental section
2.1 Material preparation
To prepare carbon-free Zn/Ti powders, 2 g mixtures of Zn
powder (<10mm,98%, Aldrich) and Ti powder (325 mesh, 99.99%,
Alfa Aesar) at atomic ratios of 1:1 (1.15 g of Zn and 0.85 g of Ti), 2:1
(1.55 g of Zn and 0.45 g of Ti), and 1:3 (0.62 g of Zn and 1.38 g of Ti)
were heat-treated at 600C for 6 h in an Ar (10 cc min1)
envi-ronment after physical mixing These mixtures (2 g) were then
placed in an 80 cm3zirconium oxide bowl with zirconium oxide
balls (diameters 3/8 in and 3/16 in.) at a ball-to-powder mass ratio
of 20:1 and sealed in an Ar atmosphere Planetary mechanical
milling (Pulverisette 5, Fritsch) was performed at room
tempera-ture at 300 rpm for 40 h (repetition of 1 h milling with 30 min
break) For the preparation of the Zn-TiC-C composite, the same
procedure was applied except that 30 wt% of carbon black (Super P,
Alfa Aesar, 99.99%) was added to a mixture of Zn/Ti at the
ball-milling stage
2.2 Material characterization
The resulting electrode powder was characterized by X-ray
diffraction (XRD) with an X-ray diffractometer with Cu Ka
(l¼ 1.5406 nm) radiation (D/MAX-2200 Rigaku, Japan) A micro Raman spectrometer (ANDOR Monora500i, 633 nm) and an X-ray photoelectron spectroscopy (XPS, Kratos AXIS Nova) were used to investigate the structure information of as-prepared powder The Brunauer-Emmett-Teller (BET) nitrogen adsorption/desorption method (ASAP 2020, Micromeritics, USA) was used to measure the specific areas A scanning electron microscope (SEM, Hitachi S4700, Japan) and a high-resolution and scanning transmission electron microscope (HRTEM and STEM, TECNAI G2F30) equipped with energy dispersive X-ray (EDX) spectrometry were used to investi-gate the morphologies, composition, and element analysis of the synthesized samples Ex situ SEM images were obtained to observe the morphology of different electrode materials before and after 50 cycles
2.3 Electrochemical measurements Electrochemical performances were measured using CR2032-type coin cells assembled in an Ar-filled glove box, in which a working electrode as an anode and a lithium foil as a counter electrode were separated by a polyethylene membrane, along with
1 M LiPF6in ethylene carbonate (EC)/diethylene carbonate (DEC) (1:1 by v/v) as an electrolyte solution The working electrode was prepared by mixing 70 wt% of as-prepared powder, 15 wt% of car-bon black, and 15 wt% of binder (polyvinylidenefluoride (PVDF)) in N-methyl-pyrrolidone solvent The mixture was then cast on a copper foil by the doctor blade method followed by drying at 70C for 12 h in vacuum Galvanostatic charge-discharge cycling tests were performed using a battery cycler (WBCS3000, WonAtech) in the range 0.01e2.0 V (vs Li/Liþ) at a current density of 0.1 A g1 The total weight of the active material consisting of pure Zn and ZnxTiy was used to calculate the gravimetric capacity The volumetric ca-pacity (with a unit of mAh cm3) was achieved using the tap density (g cm3) and measured gravimetric capacity of the powder electrode, in which the tap density was obtained from the measured volume and weight of the powder The cyclic voltam-metrogram (CV) was measured by a ZIVE MP1 multichannel elec-trochemical workstation (WonAtech) in the range of 0.01e2.0 V at a scanning rate of 0.2 mV s1 The rate capability was measured at different current densities of 0.1, 0.5, 1, and 3 A g1using a battery cycler (WBCS3000, WonAtech) Electrochemical impedance spec-troscopy (EIS) measurements were performed using a multichannel electrochemical workstation (ZIVE MP1, WonAtech) in the range
100 kHz to 100 mHz
3 Results and discussion The XRD patterns of the carbon-free Zn/Ti alloys and Zn-TiC-C are shown inFig 1aec, together with the theoretical peak positions of the Zn, Ti, TiC, Zn3Ti, and Zn0.6Ti0.4phases inFig 1d (PDF#04-0831, PDF#44-1294, PDF#32-1383, PDF#07-0098, and PDF#17-0672, respectively) For the carbon-free Zn/Ti before HEMM (i.e., only af-ter heat-treatment,Fig 1a), the diffraction peaks can be identified with four phases of Zn, Ti, Zn3Ti, and Zn0.6Ti0.4 that are well-matched with the reference peaks inFig 1d, suggesting that the introduction of thermal-treatment before HEMM facilitates the formation of Zn3Ti and Zn0.6Ti0.4 phases; however, a significant amount of Zn and Ti phases remains in the alloy All peak positions remain unchanged for carbon-free Zn/Ti after HEMM in contrast to that before HEMM, except for the decrease in the peak intensities for the Ti and Zn3Ti phases and the increase in the peak intensities for the Zn0.6Ti0.4phase (Fig 1b), indicating transformation from Ti and
Zn3Ti to the Zn0.6Ti0.4phase during HEMM (i.e., the formation of
Zn-ZnxTiy) However, the main peaks of Zn at ~39(100) and 43.2(101) indicates the broadened and decreased intensity after the HEMM
Trang 3process, revealing the reduced crystallinity of Zn during HEMM On
the other hand, the peaks observed for the Zn-TiC-C composite
(Fig 1c) are clearly distinct from those of the carbon-free Zn/Ti
sample The peaks found at ~36, 41.7, and 60.4correspond to the
theoretical peaks of the TiC phase (PDF#32-1383) with the weak Zn
peaks at ~39and 43.2(PDF#04-0831), indicating the presence of
the conductive TiC phase together with nanosized Zn in the
com-posite (i.e., the formation of Zn-TiC-C) The formation of TiC phase in
the presence of free carbon matrix to form the hybrid TiC-C matrix is
further confirmed by Raman (Fig S1) and deconvoluted XPS spectra
(Fig S2) The successful formation of the TiC phase is in good
agreement with the previous results obtained with similar
ap-proaches [33,38e40] Minor Zn3Ti peaks are also observed in the
case of Zn-TiC-C, while the peaks of Zn0.6Ti0.4disappear This could
be attributed to the priority of the TiC formation by consuming Ti and carbon rather than the production of Zn0.6Ti0.4for Zn-TiC-C The overall reactions for carbon-free Zn/Ti and Zn-TiC-C can be sum-marized as follows, and are shown inFig 1e:
Carbon free Zn=Ti alloys ðZn=TiÞ: Zn þ Ti Heattreatment/ Zn
þ Ti þ Zn3Tiþ Zn0:6Ti0:4HEMM/ Zn ZnxTiyðZn0:6Ti0:4ðmajorÞ
þ Zn3TiðminorÞÞ
(1)
Fig 1 (aec) XRD patterns of carbon-free Zn/Ti (11)-before HEMM, Zn/Ti (11)-after HEMM, and Zn-TiC-C (d) The theoretical XRD data are shown as a reference (e) Schematic diagram of the fabrication of carbon-free Zn/Ti (11) and Zn-TiC-C.
Trang 4Zn=Ti C compositeðZn TiC CÞ:Zn þ Ti Heattreatment/ Zn
þ Ti þ Zn3Tiþ Zn0:6Ti0:4HEMM/ Zn TiC C
(2)
Comparison of the XRD results for carbon-free Zn/Ti alloys with
different atomic ratios (1:1, 2:1, and 1:3, hereafter, denoted as Zn/Ti
(11), Zn/Ti (21), and Zn/Ti (13), respectively) after HEMM are shown
in Fig 1b and Fig S3 Compared to the carbon-free Zn/Ti (11)
(Fig 1b), the existence of Ti (red) and Zn0.6Ti0.4(green) phases
be-comes more pronounced in the case of the highest Ti content (ZnTi/
(13) (Fig S3b)), while the Ti (red) and Zn0.6Ti0.4(green) peaks are
significantly reduced in the case of the smallest Ti content (Zn/Ti
(21) (Fig S3a)) Because pure Ti is inactive to the Liþion storage, a
high content of Ti (i.e., for Zn/Ti (13)) can reduce the specific
ca-pacity of the electrode However, excessive Zn content (i.e., for Zn/Ti
(21)) can lead to poor cycling behavior because of the insufficient
amount of inactive components (Ti and Zn0.6Ti0.4) that cannot
sufficiently prevent the large volume changes of Zn particles during
the cycling
The SEM micrographs of the as-prepared carbon-free Zn/Ti al-loys with three different ratios (Zn/Ti (21), Zn/Ti (11), and Zn/Ti (13)) are shown inFig 2aec, together with that of the Zn-TiC-C composite (Fig 2d) Of interest is that the average particle sizes of carbon-free Zn/Ti alloys depend on the Zn content, with increasing particle size and increasing Zn content (Fig 2aec) This could be attributed to the greater probability of melting and agglutability of
Zn (melting temperature of 420C) than Ti (melting temperature of
1668C) during HEMM In addition, the significant reduction in the average particle size for Zn-TiC-C is because of the continuous fracture and welding of powders with significantly strong TiC phases [7,33] Although the average particle sizes are relatively large (mostly< 5mm) for both Zn/Ti (11) and Zn/Ti (13) compared with that of Zn-TiC-C (mostly< 1mm), they are still significantly smaller than that of Zn/Ti (21) (primarily> 10mm) (Fig 2e), indi-cating the greater surface area of Zn/Ti (11) and Zn/Ti (13) than that
of Zn/Ti (21) Also, from the BET analyses for pure Zn, Zn/Ti (11), and Zn-TiC-C inFig 2f, Zn-TiC-C shows the highest surface area, fol-lowed by Zn/Ti (11) and pure Zn
Fig 3shows the HRTEM and STEM with EDS mapping images of
Fig 2 SEM images of (a) Zn/Ti (21), (b) Zn/Ti (11), (c) Zn/Ti (13), and (d) Zn-TiC-C (e) Particle size distribution of carbon-free Zn/Ti alloys and Zn-TiC-C composite (f) BET results of
Trang 5each element (Ti: red; Zn: green; and C: blue) for carbon-free Zn/Ti
(11) alloy (Fig 3a and b) and Zn-TiC-C composite (Fig 3c and d) As
can be seen in the HRTEM images, the carbon-free Zn/Ti (11) alloy
(Fig 3a) displays Zn (100), Zn3Ti (100), and Zn0.6Ti0.4(100) phases
corresponding to lattice plane separations of ~0.23, 0.4, and
0.32 nm, respectively, which are homogeneously dispersed in the
alloy However, the Zn-TiC-C composite (Fig 3c) exhibits the lattice
fringes of Zn (~0.23 nm) and TiC (~0.21 nm) corresponding to the
(100) plane for Zn and (200) plane for TiC, which are consistent
with the XRD results Additionally, the STEM and EDS mapping
images of both carbon-free Zn/Ti (11) (Fig 3b) and Zn-TiC-C further
confirm the homogeneous distribution of Zn and Ti throughout a
large area The crystallite sizes of Zn, Zn3Ti, and Zn0.6Ti0.4phases are
estimated to be smaller than 10 nm, which is in good agreement
with the domain sizes obtained from the Scherrer equation based
on XRD analysis (d¼ 0.9 l)/(b cos(q)), wherelis X-ray
wave-length (0.15406 nm),bis the line broadening at half the maximum
intensity (FWHM), andqis the Bragg angle (where 2qs are ~39,
22.6, and 27.6 for Zn (110), Zn3Ti (100), and Zn0.6Ti0.4 (100),
respectively)
Fig 4a shows the initial charge/discharge profiles of carbon-free
Zn/Ti (11), Zn-TiC-C, and pure Zn electrodes at a constant current
density of 0.1 A g1 As can be seen inFig 4a, these electrodes
exhibit initial discharge/charge capacities of ~607/535, 574/411, and
488/228 mAh g1/mAh g1corresponding to the initial coulombic
efficiencies (ICEs) of ~88%, 72%, and 47%, respectively The initial
capacity loss is caused primarily by the irreversible reaction of Zn
with Li and a subreaction between electrolytes and the electrode
surface Therefore, the lowest ICE of pure Zn electrode could be
attributed to the absence of the buffering matrix that can mitigate the volume changes of Zn However, the introduction of buffering
ZnxTiy, or the TiC-C phase for carbon-free Zn/Ti (11) alloy or for Zn-TiC-C composite, significantly suppresses the volume change of active Zn, resulting in improved ICEs of these electrodes On the other hand, the initial capacity loss of Zn-TiC-C is greater than that
of carbon-free Zn/Ti (11) because of the irreversible reaction of Li with free carbon (that could be confirmed by Raman and XPS re-sults inFig S1andFig S2, respectively) and the subreaction that are significantly decreased in the case of carbon-free Zn/Ti (11) After the 1st cycle, excellent CEs are observed for the carbon-free Zn/Ti (11) (with 95% and 96%), for Zn-TiC-C (with 97% and 95%) for the second and third cycles, respectively, as shown in Fig S4a and
Fig S4b, which could be attributed to the existence of stable ZnxTiy phases and TiC-C hybrid matrix that accommodate volume changes
of Zn particles during repeated cycling as compared to the poor CEs for pure Zn electrode (with 93% and 88% for the second and third cycle, respectively (Fig S4c) The initial discharge capacities of all samples are typically greater than their calculated theoretical ca-pacities This is associated with the formation of the solid electro-lyte interface (SEI) layer on the surface of the Zn particles and side reactions from impurities during the initial stage of the electro-chemical reaction [7,33]
To better understand the electrochemical reaction mechanisms
of Zn, Zn-TiC-C, and carbon-free Zn/Ti (11), cyclic voltammograms (CVs) were plotted for thefirst three cycles, as shown inFig 4bed During thefirst discharge process of the pure Zn electrode (Fig 4b), the small peak at ~0.65 V corresponds to the formation of the SEI layer on the surface of Zn, while the two larger peaks at ~0.35
Fig 3 High-resolution TEM and STEM images along with EDS mapping images for each element of (aeb) carbon-free Zn/Ti (11) alloy and (ced) Zn-TiC-C composite.
Trang 6and< 0.2 V are associated with the sequential lithiation reactions of
Li with Zn to form the LiZn phase (Zn/ LiZn4/ LiZn) [27,33] In
the 1st charge, two large peaks at ~0.28 and ~0.33 V and two small
peaks at ~0.55 and ~0.7 V are observed, which are matched with a
series of delithiation processes (LiZn/ LiZn2/aLi2Zn5/aLiZn4
/ Zn) [27] In the subsequent discharge/charge processes, the SEI
peaks disappear and the lithiation/delithiation peaks remain sharp
except for the significant reduction in their intensities In the case of
Zn-TiC-C, similar behaviors to the Zn electrode are exhibited, except
for an additional peak at ~0.9 V corresponding to the formation of
an SEI layer on the surface of the carbon [41] (Fig 4c) The initial
capacity of Zn-TiC-C is greater than that of pure Zn (574 mAh g1
compared to 488 mAh g1), in all probability because of the greater
surface area, as shown in Fig 2f Although the introduction of
carbonaceous materials typically decreases the overall Li-storage
capacity because of the reduced content of active material (Zn in
this case), the pure Zn electrode showed the lower specific capacity
than Zn-TiC-C due to much smaller surface area (2.7 m2g1for pure
Zn compared to 9.1 m2g1for Zn-TiC-C) and much bigger particle
sizes that prevent the complete reaction between active Zn and Li
ions, thereby reducing the initial Li-storage capacity On the other
hand, upon comparing Zn/Ti (11)-C and Zn/Ti (11), the capacity of
Zn/Ti (11) is higher than that of Zn/Ti (11)-C in which the
concen-tration of active material might have played a major role in
increasing the capacity because the surface area of Zn/Ti (11) was
lower than that of Zn/Ti (11)-C (Fig 2f) Overall, the capacity
contribution from carbon should be understood considering many
different factors such as surface area (or particle size) and
con-centration of active material (or theoretical capacity) together
However, all lithiation/delithiation peaks become smeared as the
Zn nanocrystallites are well dispersed in the TiC-C matrix during
the HEMM [7] In addition, from the 2nd cycle the SEI peak at ~0.9 V
disappear and all other peaks retained their positions and
intensities, indicating the stabilized electrochemical reactions of Zn with Liþbecause of the existence of the TiC-C matrix [7,33,38e40] Remarkably, in the case of the carbon-free Zn/Ti (11) electrode (Fig 4d), apart from one peak that appeared at ~1.1 V that could be attributed to the SEI layer formation on the surface of ZnxTiyphases, all other peaks are observed to be in good agreement with the case
of pure Zn because of the presence of active Zn particles in the
ZnxTiymatrix After the 1st cycle, the reduction peak at ~0.35 V is shifted toward a higher potential because of the reduced polari-zation during the discharge process, whereas the oxidation peaks are typically overlapped with little change in their intensities, indicating the highly stabilized electrochemical reactions after the 1st cycle
For a better understanding of the Li storage mechanism of the Zn/Ti (11) electrode during cycling, ex situ XRD measurements were conducted by opening the cells at the fully lithiated and delithiated along with pristine stage, as shown inFig 5a For pristine Zn/Ti (11) electrode, the crystalline phases observed (Zn, Zn0.6Ti0.4, and Zn3Ti, together with two prominent peaks of copper foil) are typically consistent with the XRD results for as-prepared powder (Fig 1b), except for their reduced intensities as the ex situ XRD measure-ments were performed on the electrode material comprising additional conductive carbon and PVDF binder as well as active material (Zn/Ti (11)) In the fully lithiated at 0.01 V,five new peaks
at ~21, 24, 48, 60.5, 65.5, corresponding to LiZn phase, were observed together with the remained peaks of Zn0.6Ti0.4, Zn3Ti, and
Cu, while the main peak of Zn at ~43.5 showed the significant reduction in intensity This indicates that most of Zn reacted with Li
to form Li-Zn alloy during the discharge process, which are in good agreement with results of Hwa et al [27] On the other hand, after complete delithiation at 2.0 V, the peak intensity of Zn at ~43.5 recovered to approximately that of the pristine stage In addition, the peaks of the Li-Zn alloy disappears at the full-charged stage,
Fig 4 (a) Initial voltage profile and (bed) cyclic voltammetry plots of pure Zn, Zn-TiC-C, and carbon-free Zn/Ti (11).
Trang 7which demonstrates the complete reversibility of LiZn to the Zn
phase In addition, the Zn0.6Ti0.4 and Zn3Ti peaks remains
un-changed throughout the discharge/charge process as well as
pris-tine stage, indicating electrochemical inertness and acting as a
conductive matrix for Zn Ex-situ HRTEM image and SAED pattern of
Zn/Ti (11) at fully lithiated process was further investigated in
Fig 5b This results display LiZn (311) phase corresponding to the
lattice plane separation of ~0.19 nm beside the presence of Zn0.6Ti0.4
phase ((311) plane corresponding to ~0.32 nm), which are fully
consistent with ex-situ XRD result inFig 5a
Fig 6a compares the cyclic performances of the carbon-free Zn/
Ti (11), Zn-TiC-C and pure Zn electrodes based on the gravimetric
capacities andFig 6b illustrates the volumetric capacities of the
carbon-free Zn/Ti (11), Zn-TiC-C and pure Zn electrodes Despite
their high initial gravimetric capacities (607, 574, and 488 mAh g1
for carbon-free Zn/Ti (11), Zn-TiC-C and pure Zn, respectively), they
exhibit distinct behavior after the second cycle The specific
ca-pacity rapidly decays to less than 100 mAh g1after 15 cycles for
pure Zn electrode, which could be ascribed to mechanical cracking
and crumbling caused by large volume changes during repeated
cycling In contrast, when the TiC-C matrix is introduced, the
Zn-TiC-C electrode exhibits excellent cyclic stability, as even after
350 cycles the specific capacity remained at ~304 mAh g1(94% of
capacity retention compared to the capacity at the 2nd cycle) The enhanced cyclic stability could be attributed to the uniform dispersion of Zn particles in the TiC-C conductive matrix that suppresses particle agglomeration and accommodates the volume expansion of Zn particles during Li alloying [33,38e40,42] Of
sig-nificance is that the formation of the ZnxTiyphase in the case of carbon-free Zn/Ti (11) also contributes to the enhanced cyclic sta-bility because of its mechanical stasta-bility and cohesive properties that could buffer the volume changes of Zn particles during cycling [37], and could even increase the specific capacity compared to that
of Zn-TiC-C because of the absence of carbon, as discussed earlier Therefore, the carbon-free Zn/Ti (11) electrode exhibits the superior cyclic performance with 350 mAh g1of gravimetric capacity after
350 cycles, which is significantly greater than that of the Zn-TiC-C composite The XPS result inFig S5presents the signal from Zn deconvoluted into two peaks located at ~1044.8 and ~1021.7 eV corresponding to the Zn 2p1/2and Zn 2p3/2, respectively, confirming the pure Zn In addition, there are two small peaks detected at ~975 and ~998 eV which might be ascribed to the Zn 2p in the ZnxTiy
phases (circle inFig S5a), while two signals from Ti 2p1/2and Ti 2p3/2appeared at ~458 and 464 eV, which are higher than those
Fig 5 Ex situ XRD patterns of carbon-free Zn/Ti (11) electrode at fully lithiated and delithiated state along with pristine stage (b) Ex-situ HRTEM image and SEAD patterns of carbon-free Zn/Ti (11) at fully lithiated state.
Trang 8from metallic Ti 2p peaks (~454 and 460 eV), indicating the
for-mation of ZnxTiyrather than pure Ti [33] Owing to the small peaks
of Zn 2p in ZnxTiy (Fig S5a), it could be effectively considered
negligible Therefore, the mass percentage of pure Zn was
esti-mated based on the deconvoluted Zn 2p and Ti 2p peaks, which
gives 67.54% of pure Zn Therefore, the theoretical capacity of
carbon-free Zn/Ti (11) could be calculated based on wt% of pure Zn
in Zn/Ti alloy; that is ~277 mAh g1 This theoretical capacity of Zn/
Ti alloy is lower than the specific capacity (350 mAh g1) from
electrochemical results, which might be attributed to the
Li-adsorption in the structure of ZnxTiyphases Due to the formation
of Zn3Ti and Zn0.6Ti0.4phase with lattice plane separation of 0.4 nm
and 0.32 nm, respectively, which are similar or even higher than
that of graphite (~0.34 nm), the Li-storage mechanisms based on
Zn3Ti and Zn0.6Ti0.4edges surface without any change in structure
of ZnxTiycould be expected just as in the mechanism of excess
Li-storage sites in carbon materials that have been investigated in
previous reports [43e46] In addition, the tap density of
carbon-free Zn/Ti (11) increases significantly up to ~3 g cm3 compared
to ~1.25 g cm3for Zn-TiC-C (Table S1), resulting in a significant
volumetric capacity of 1064 mAh cm3after 350 cycles which is
over two-fold higher than the volumetric capacities of Zn-TiC-C
(430 mAh cm3) and pure Zn (525 mAh cm3) (Fig 6b)
The rate capability and normalized capacity retention of these
electrodes at various current densities from 0.1 to 3 A g1 are
shown inFig 6c and d The pure Zn electrode exhibits a poor rate
capability and delivers a charge capacity of ~231 mAh$cm3 at a
current density of 3 A g1, corresponding to 29% capacity retention
compared to the capacity at 0.1 A g1 In addition, when ZnxTiyor
TiC-C is introduced, the rate capability is significantly improved to
~900 mAh$cm3and ~310 mAh cm3of reversible charge capac-ities, respectively, corresponding to charge capacity retentions of 81% for both carbon-free Zn/Ti (11) and Zn-TiC-C The excellent performance of these electrodes could be attributed to the high degree of incorporation of the conductive hybrid TiC-C matrix as well as the ZnxTiyintermetallic phases, which promote electron transport in the electrodes and act as an effective buffer that mit-igates stress and strain during the cycling In addition, the greater capacity of the carbon-free Zn/Ti (11) electrode than Zn-TiC-C is consistently confirmed for all current densities (Fig 6c), which demonstrates the superiority of the ZnxTiyalloy to TiC-C When the rate capability between different metal ratios (Zn:Ti) is compared, the poor performance of Zn/Ti (21) is observed (Fig S6), which could be attributed to the excessive amount of Zn in Zn/Ti inter-metallic compounds that leads to the difficulty in forming the
Zn0.6Ti0.4matrix and the greatest particle size (Fig 2a) than that observed at other ratios (1:1 and 1:3,Fig 2b and c, respectively) However, in spite of the high cyclic stability until 300 cycles, Zn/Ti (1:3) exhibits significantly lower capacities (~507 mAh cm3after
300 cycles) than Zn/Ti (1:1) (~1062 mAh$cm3 after 300 cycles), indicating that an excessively high Ti content in Zn/Ti also leads to a reduced specific capacity of the electrode
To further confirm the enhanced electrical conductivity of carbon-free Zn/Ti (11) in comparison with the Zn-TiC-C composite and pure Zn electrodes, we performed EIS measurements for the three different electrodes before and after 50 cycles at the same current density As shown inFig 7along with equivalent circuit model, the depressed semicircle in the medium-to-high-frequency region consists primarily of the interfacial charge-transfer imped-ance (Rct), although the high-frequency SEIfilm impedance (Rs)
Fig 6 (a) Long-term cycling performance of carbon-free Zn/Ti (11), Zn-TiC-C, and pure Zn from 0.01 to 2.0 V vs Liþ/Li, (b) Volumetric capacities calculated based on mass and tap density of active material, (c) Rate capability of carbon-free Zn/Ti (11), Zn-TiC-C, and pure Zn at various current densities of 0.1, 0.5, 1,3, and 0.1 A g1, and (d) Normalized charge capacity retention).
Trang 9also contributes to its electrochemical impedance The charge
transfer resistance (Rct) and SEIfilm resistance (Rs) are estimated by
applying the simplified equivalent circuit (as shown inFig 7c) and
fitting this model to the Nyquist plots where the diameters of the
first and second semicircles correspond to Rsand Rct, respectively
[39] The values of Rctof electrodes after 50 cycles are 80, 131, and
213Ufor carbon-free Zn/Ti (11), Zn-TiC-C, and pure Zn, respectively,
which are much smaller than those of before cycle electrodes of
169, 259, 265U, respectively This could be attributed to the
acti-vation process of those electrodes after cycling compared to fresh
cells On the other hand, as compared to Zn-TiC-C and pure Zn
electrodes, carbon-free Zn/Ti (11) shows the lowest charge-transfer
impedance in both cases of before and after cycles These results indicate that the introduction of ZnxTiy as a conductive matrix provides effective electron pathways into the active particles, thereby enhancing the electronic conductivity However, the pure
Zn electrode shows significantly greater charge-transfer resistance, resulting from the large volume changes during repeated cycling that leads to electrode pulverization and loss of electrical contacts with the current collector after 50 cycles These results are consistent with the long-term cyclic performance and rate capa-bility, as shown inFig 6
Finally,Fig 8shows the ex situ SEM images of carbon-free Zn/Ti (11) and pure Zn electrode before and after 50 cycles It can be seen
Fig 7 EIS spectra of carbon-free Zn/Ti (11), Zn-TiC-C, and pure Zn (a) before and (b) after 50 cycles along with equivalent circuit model.
Fig 8 Ex situ SEM images of pristine and 50 cyclized electrodes of (aeb) carbon-free Zn/Ti (11) and (ced) pure Zn.
Trang 10that the original morphology has been primarily maintained for the
carbon-free Zn/Ti (11) electrode after 50 cycles, while the pure Zn
electrodefilm exhibits significant aggregations, even the
peeling-off from the current collector, resulting in the loss of electrical
contacts with the current collector, poor reversibility, and
eventu-ally rapid capacity decay This result explains the excellent
elec-trochemical performances of carbon-free intermetallic Zn/Ti (11)
alloys over pure Zn anodes for LIBs
4 Conclusions
The performance enhancement of carbon-free intermetallic zinc
titanium alloy anode was demonstrated for LIBs for thefirst time
The primary reason for this improved performance is the presence
of stable and cohesive ZnxTiyintermetallic phases that are formed
via thermal-treatment followed by HEMM These phases act as an
effective buffering matrix for Zn during the cycling The formation
of the intermetallic ZnxTiyphase without carbon contributed to the
superior cycle life and rate capability because of the buffering role
against Zn volume changes and the conductivity enhancement In
addition, the Zn-ZnxTiyelectrode demonstrates improved
gravi-metric and volugravi-metric capacity because of no capacity contribution
from carbon and high tap density Overall, the carbon-free
inter-metallic Zn/Ti (11) alloy can be seen as a new promising anode for
next-generation energy storage applications
Acknowledgments
This research was supported by the Basic Science Research
Program through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education
(NRF-2016R1D1A1B03931903) This work was supported by the Korea
Institute of Energy Technology Evaluation and Planning (KETEP)
and the Ministry of Trade, Industry& Energy (MOTIE) of the
Re-public of Korea (No 20162010104190)
Appendix A Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.electacta.2019.01.182
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